1. Field of the Invention
The present invention relates to a method of and an apparatus for cleaning a workpiece, and more particularly to a method of and an apparatus for cleaning a workpiece that is required to have a high degree of cleanliness, e.g., a semiconductor wafer, a glass substrate, a liquid crystal display, or the like.
2. Description of the Prior Art
As semiconductor devices have become more highly integrated in recent years, circuit interconnections on semiconductor substrates have become finer and the distance between each circuit interconnections have become smaller. When semiconductor wafers are processed, small particles such as particles of semiconductor material, dust particles, crystalline protrusive particles, or the like often tend to be attached to the semiconductor wafers being processed. If a particle greater than the distance between interconnections exists on a semiconductor substrate, then the particle will short-circuit interconnections on the semiconductor substrate. Therefore, any undesirable particles on a semiconductor substrate have to be sufficiently smaller than the distance between interconnections on the semiconductor substrate. Such a problem and a requirement hold true for the processing of other workpieces including a glass substrate to be used as a mask, a liquid crystal display, and so on. To meet the above requirement, there have been practiced various cleaning procedures for removing fine particles or submicron particles from semiconductor wafers.
For example, heretofore one practice has been to use a brush of nylon, mohair or the like, or a sponge of polyvinyl alcohol (PVA) to scrub a surface of a semiconductor wafer. This process is called a scrubbing cleaning process. Further, there have been other practices, one of which is an ultrasonic cleaning process in which water having ultrasonic vibrational energy applied thereto is supplied to a surface of a semiconductor wafer, and another of which is a cavitation jet cleaning process in which high pressure water containing cavitation (i.e., cavitation bubbles) therein is supplied to a surface of a semiconductor wafer. Also, a cleaning process which combines two or three of the above processes is known in the art. A scrubbing cleaning process using a brush of nylon, mohair, or the like is effective to remove particles having diameters of 1 .mu.m or larger from semiconductor wafers. However, such a scrubbing process fails to produce an appreciable cleaning effect on submicron particles smaller than such particle sizes, and adversely produces scratches over a surface of the semiconductor wafer. Further, when a number of semiconductor wafers are cleaned by the brush, particles are attached to the brush which in turn contaminates subsequent semiconductor wafers.
Particles having diameters of submicron size can be removed from semiconductor wafers by another scrubbing process sing a sponge of polyvinyl alcohol (PVA). The sponge of PVA has a tendency to entrap particles therein. This scrubbing process is, however, ineffective where a certain amount of particles are entrapped by the sponge. That is, the sponge of PVA does not have a long service life without suitable regeneration.
As described above, in case of a scrubbing cleaning process using a brush or a sponge, the brush or the sponge is contaminated during the cleaning process. Therefore, in the case where a semiconductor wafer to which a great number of particles are attached, as in the case of a semiconductor wafer which has been polished, is cleaned using the brush or the sponge, a brush or the sponge must be cleaned or replaced, resulting in imposing the serious burden on the operator.
Further, in case of cleaning a semiconductor wafer having a circuit pattern thereon, as particles are smaller and gaps into which particles are entered are smaller, the semiconductor wafer cannot be cleaned only by the scrubbing cleaning process.
On the other hand, an ultrasonic cleaning process and a cavitation jet cleaning process do not require cleaning by a brush or a sponge, and can remove some particles which cannot be removed by the scrubbing cleaning process.
Further, an ultrasonic cleaning process and a cavitation jet cleaning process are effective up to a degree of several ten or several hundred particles on a surface of a semiconductor wafer having a diameter of eight inches (200 mm).
However, in case of a semiconductor wafer which has been subjected to CMP (chemical mechanical polishing) process, the semiconductor wafer carries hundreds of thousands of particles. Thus, the ultrasonic cleaning process and the cavitation jet cleaning process are ineffective, because they can reduce the number of particles only to a degree of several thousands or several tens of thousands of particles.